1. Field of the Invention
The present invention relates to capstans for proof testing, and in particular to capstans for proof testing that reduce shear imposed on an optical waveguide fiber and for reducing deformation of the coating of the optical waveguide fiber during the proof testing process.
2. Technical Background
Capstan assemblies as used in the manufacture of optical waveguide fibers are typically used to draw the optical fibers from glass blanks that are mounted within draw towers, and/or for proof testing of the optical fiber, also known as fiber screening. For consistency, the term proof testing will be used herein.
Capstan assemblies typically include a capstan pulley and a capstan belt between which the optical fiber is positioned. As the capstan pulley is rotated, the friction generated between the capstan pulley, the optical fiber and the capstan belt pulls or draws the optical fiber from the associated glass blank through a series of related operations such as coating and sizing steps. When used in tandem, capstan assemblies can also be used to test the proof strength of the optical fiber by placing a tensile stress thereon.
While stressing the fiber is necessary to test for proof strength, the shear forces imposed on the coating during the process of applying the required proof stress to the fiber should be limited to avoid the negative effect of damaging the coating.
When used for proof testing, capstans typically have some region in the arc of contact of the fiber with the capstan pulley where the optical fiber is slipping relative to the capstan, thereby defining an arc of slip. The region about the capstan pulley that the optical fiber is not slipping is defined as the arc of adhesion. This slip is actually desirable to minimize shear, as shear is reduced, or its increase is limited, once slip occurs. Therefore, earlier slip results in less shear. For a given set of capstan pulley/belt materials, the slip is a function of the lateral load on the fiber due to the belt and the fiber tension and varies inversely thereto.
In the prior art, the lateral load on a fiber is reduced by reducing the belt tension. This solution poses some practical problems, including 1) reducing belt tension to the point that no xe2x80x9carc of adhesionxe2x80x9d occurs between the fiber and the capstan, thus resulting in catastrophic slip and compromising proof testing of the optical fiber, and 2) belt tracking (i.e., its propensity for side-to-side motion) as well as lateral fixity of the fiber (i.e., how well the fiber position beneath the belt is maintained against side-to-side motion).
At the other extreme, increasing the lateral load on the optical fiber, thereby reducing slip, by increasing the belt tension poses additional problems. These include coating damage, which is manifested as cohesive failure at or near the coating/glass interface. More specifically, the lateral load as supported by the fiber can cause permanent or irrecoverable deformation of the coating surrounding the optical fiber.
A capstan assembly that provides sufficient tension during drawing and/or proof testing of an optical fiber while reducing unwanted shear on the optical fiber and deformation of the associated fiber coating would be advantageous for ensuring proof strength and coating integrity.
One aspect of the present invention is to provide an apparatus for proof testing an optical waveguide fiber that includes a first capstan pulley defining a first outer diameter and a continuous first capstan belt under tension and in contact with the first outer diameter of the first capstan pulley, wherein the contact between the first capstan belt and the first capstan pulley define a first arc of contact, and the first capstan belt defines a first belt width, a first belt thickness and a first belt length. The apparatus further includes a second capstan pulley defining a second outer diameter and a continuous capstan belt under tension and in contact with the outer diameter of the second capstan pulley, wherein the contact between the second capstan belt and the second capstan pulley defines a second arc of contact, the second capstan belt defines a second belt width, a second belt thickness and a second belt length. The first arc of contact is preferably equal to or greater than about 105xc2x0 and the second arc of contact is at least 20xc2x0 greater than the first arc of contact. The first arc of contact can be as low as 90xc2x0.
Another aspect of the present invention is to provide an apparatus for proof testing an optical waveguide fiber that includes a first capstan pulley defining a first outer diameter and a continuous capstan belt under tension and in contact with the first outer diameter of the first capstan pulley, wherein the contact between the first capstan belt and the first capstan pulley defines a first arc of contact, and the first capstan belt defines a first belt width, a first belt thickness and a first belt length. The apparatus further includes a second capstan pulley defining a second outer diameter and a continuous second capstan belt under tension and in contact with the outer diameter of the second capstan pulley, wherein the contact between the second capstan belt and the second capstan pulley defines a second arc of contact, and the second capstan belt defines a second belt width, a second belt thickness and a second belt length. The first width of the first capstan belt and the second width of the second capstan belt each have a lateral modulus of elasticity, and the first length of the first capstan belt and the second length of the second capstan belt each have a longitudinal modulus of elasticity, and the lateral modulus of elasticity of each belt is between about ⅕ and about ⅙ the longitudinal modulus of elasticity of each belt.
Yet another aspect of the present invention is to provide an apparatus for proof testing an optical fiber that includes a first capstan pulley defining a first outer diameter and a continuous first capstan belt under tension and in contact with the first outer diameter of the first capstan pulley, wherein the contact between the first capstan belt and the first capstan pulley defines a first arc of contact and the first capstan belt defines a first belt width and a first belt thickness. The apparatus further includes a second capstan pulley defining a second outer diameter and a continuous second capstan belt under tension and in contact with the outer diameter of the second capstan pulley, wherein the contact between the second capstan belt and the second capstan pulley defines a second arc of contact and the second capstan belt defines a second belt width and a second belt thickness. The second arc of contact being at least 20xc2x0 greater than the first arc of contact.
Another aspect of the present invention is to provide a method for proof testing an optical waveguide fiber including providing an optical fiber, drawing the fiber through a first rotating capstan with a first arc of contact, and drawing the fiber through a second rotating capstan with a second arc of contact that is at least 20xc2x0 greater than the first arc of contact.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.